STUDY OF SLOPE STABILITY IN
LIGNITE MINES

By Mamta Jaswal (090610122047)
Nilay J. Patel (090610122029) Ashish D. Patel (090610122026)
Satish B. Patel (090610122013)

Supervised by:
Prof. Rajesh Arora
M. Tech.

A project part-1 submitted to
Gujarat Technological University
In partial fulfilment of
The Requirements for the
Degree of Bachelor of Engineering
In Mining Engineering

October 2012

CERTIFICATE

This is to certify that Ms. Mamta Jaswal of BE. Semester VII (Mining engineering) has completed her one full semester on project work Titled “STUDY OF SLOPE STABILITY IN LIGNITE MINES” Satisfactorily in partial fulfilment of requirement of Bachelor of Engineering In Mining Engineering, Gujarat Technology University, Ahmedabad in the year 2012.

Date: / / 2012

Place: Palanpur

Internal Guide Prof. V.J. Sharma
Prof. Rajesh Arora Head of Department (Mining engineering)

Seal of institute

CERTIFICATE

This is to certify that Mr. Nilay J. Patel of BE. Semester VII (Mining engineering) has completed his one full semester on project work Titled “STUDY OF SLOPE STABILITY IN LIGNITE MINES” Satisfactorily in partial fulfilment of requirement of Bachelor of Engineering In Mining Engineering, Gujarat Technology University, Ahmedabad in the year 2012.

Date: / / 2012

Place: Palanpur

Internal Guide Prof. V. J. Sharma
Prof. Rajesh Arora Head of Department (Mining engineering)

Seal of institute

CERTIFICATE

This is to certify that Mr. Ashish D. Patel of BE. Semester VII (Mining engineering) has completed his one full semester on project work Titled “STUDY OF SLOPE STABILITY IN LIGNITE MINES” satisfactorily in partial fulfilment of requirement of Bachelor of Engineering In Mining Engineering, Gujarat Technology University, Ahmedabad in the year 2012.

Date: / / 2012

Place: Palanpur

Internal Guide Prof. V. J. Sharma
Prof. Rajesh Arora Head of Department (Mining engineering)

Seal of institute

CERTIFICATE

This is to certify that Mr. Satish B. Patel of BE. Semester VII (Mining engineering) has completed his one full semester on project work Titled “STUDY OF SLOPE STABILITY IN LIGNITE MINES” Satisfactorily in partial fulfilment of requirement of Bachelor of Engineering In Mining Engineering, Gujarat Technology University, Ahmedabad in the year 2012.

Date: / / 2012

Place: Palanpur

Internal Guide Prof. V. J. Sharma
Prof. Rajesh Arora Head of Department (Mining engineering)

Seal of institute
ACKNOWLEDGEMENT
We are glad to show our gratitude towards various people, who directly or indirectly contributed in the development of this work and who influenced our thinking, acts and behaviour during the course of study. We are thankful to Mr. Neeraj Pareek, our external guide for his support, cooperation, and motivation provided to us during the training for constant inspiration, presence and blessings. We also extend our sincere appreciation to Prof. Rajesh Arora, our internal guide who provided her valuable suggestions and precious time in accomplishing our project report. We express my sincere gratitude to Mr. V.J. Sharma worthy Head of the department for providing us an opportunity to present this paper. Lastly, we would like to thank the almighty and our parents for their moral support and our friends with whom we shared our day-to-day experience and received lots of suggestions that improved our quality of work.

Table of Contents
Title Page………………………………………………….……………………………………i
Certificate Page……………………………………………………………………….………..ii
Acknowledgement……………………………………………………………….……………vi
Table of Contents…………………………………………………………………….….……vii
List of figures…………………………………………………………………………...….......x
List of Tables………………………………………………………………………………….xi
Abstract……………………………………………………………………………….………xii
Chapter-1 Introduction………………………………………………………...…..…….….....1 1.1 Overview…………………………………………………………………………….…1 1.2 Objectives……………………...……………………………………………..……..…1 1.3 Problem Summary……………………………………………………….……….........1 1.4 Panandhro Lignite Project ............................................................................................2
Chapter-2 Literature Review……………………………………………………………....…...5 2.1 Open pit slopes- An introduction………...……………………………………………5 2.2 Slope stability…………………………………………………………………………..5 2.2.1 Slope Geometry………..…………………………………………………….……6 2.2.2 Geological Structure……………...………………………………………….……6 2.2.3 Lithology………………………………………………………………………..…7 2.2.4 Ground Water……………………………………………………………………..7 2.2.5 Mining methods and Equipments……………………………………………...…8 2.2.6 Dynamic Forces…………………………………………………………….……..8 2.2.7 Cohesion……………………………………………………………………….….9 2.2.8 Angle of Friction………………………………………………………..….……10 2.3 Types of Failure…………………………………………………………………..….10 2.3.1 Planar failure………………………………………………………………...…..10 2.3.2 Wedge failure………………………………………………………………...….12 2.3.3 Circular failure ……………………….…...………………………….……….…13
2.3.3.1 Types of Circular failure……………………………………………………14 2.3.4 Two Block failure………………………………………………………….…….14 2.3.5 Toppling failure………………………………………………………….………15
2.3.5.1 Types of Toppling failure…………………………………………………..15 2.4 Factors to be considered in assessment of stability……..……………………………16 2.4.1 Ground investigation ………………………………………………………….16 2.4.2 Most critical failure surface………….……………………………..…..….….17 2.4.3 Tension Crack…………….……….…………………………………......…...17 2.4.4 Submerged slopes………………………………….………………….…….17 2.4.5 Factor of safety….…………………………………………….…..…….…....18 2.4.6 Progressive failure………………………….…….….…….…….....……...…18 2.4.7 Pre-existing failure …………………………..……..….……......….…………19
2.5 Methods of analysis………………………………………………………………….19
2.5.1 Wedge failure analysis……………………………………………………..…….19
2.5.1.1 Spherical projection solution using factor of safety…………………......19
2.5.1.2 Chart solution……………………………………………………………….20 2.5.1.3spherical Projections Solutions using Probabilistic Approach…………..…...20

2.5.2 Circular Failure Analysis……………………………………………………….20 2.5.2.1 Method of Slices……………………………………………………………..20 2.5.2.2 Modified Method of Slices…………………………………………………..20 2.5.2.3 Simplified Method of Slices…………………………………………………21 2.5.2.4 Friction Circle Method………………………………………………………21 2.5.2.5 Taylor’s stability number……………………………………………………21 2.5.3 Two Block Failure Analysis……………………………………………………..21 2.5.3.1 Stereographic Solution………………………………………………………21 2.5.4 Toppling Failure Analysis……………………………………………………….21 2.5.5 Other methods of analysis……………………………………………………….22 2.5.5.1 Limit Equilibrium Method………………….……………….……………...22 2.5.5.2 Stress Analysis method…………………………….…….………….………22
Conclusion………………………………………………………………..……………………23
References……………………………………………………………………………..….……24

List of figures
Figure 1.1 Slope failure at Panandhro Lignite Project…………………………,..…………2
Figure 2.1 Bench, Ramp, Overall angle and their respective angles...................................4
Figure 2.2 Plane failure…………………………………………………………………..……8
Figure 2.3 Wedge failure…………………………………………………………………..…..9
Figure 2.4 Three-dimensional failure geometry of a rotational shear failure……………..…10
Figure 2.5 Toppling failure…………………………………………………………………...11

List of tables
Table 2.1 Guidelines for equilibrium of a slope……………………………………………...13

ABSTARCT Slope stability analysis forms an integral part of the open cast mining operations during the life cycle of the project. Most of the design methods are purely based on the field experience, rules of thumb followed by sound engineering Judgement. The number of operating opencast mines is increasing in India as well as in Gujarat. During our training session in GMDC (Gujarat Mineral Development Corporation) Lignite Project Panandhro, we noticed severe Problem of slope failure. The project addresses the issues related to slope failures and finds out the factors causing it. Slope failures causes’ loss of production, extra stripping ratio, coat of recovery, handling of failed material, dewatering pits and sometimes lead to mine abandonment/premature closure. Study incorporates design of slope taking into consideration are geotechnical properties of strata and suggest remedial measures to mitigate slope failures in lignite mines which can be useful to various lignite mines in Gujarat.

Key words- slope stability, type of failures, rock mass strength, open pit mining.
Chapter -1 Introduction
1.1 Overview: Slope stability analysis forms an integral part of the open cast mining operations during the life cycle of the project. Slope failure is one of the most commonly occurring problems in lignite mines in Gujarat. There are various factors contributing failure of slope designs. Slope failures have to be mitigated due to severe losses caused due to this phenomenon. In Gujarat, numbers of lignite mines as well as other open cast mines are steadily increasing due to low gestation period, higher productivity and quick rate of investment. Here, stability of slopes stands as one of the most important aspect. Design of the final pit limit should be prepared such that it can minimise risk of slope failures. It is not governed by the ore grade distribution and the production cost but also the overall rock mass strength and stability.
1.2 Objectives:
The objective of this project is- (i) Understanding the different types and modes of the slope failures. (ii) Finding reasons behind slope failures and methods to minimise them.
1.3 Problem Summary: Lignite and limestone are the minerals which are mined at the LIGNITE PROJECT PANANDHRO. Panandhro lignite mine is the open surface mine which has about 1719 hectares total lease area. The project was start in 1974. The total reserve of lignite at Panandhro was 112 million tones and the total produced lignite was 98 million tones.(up to dec.’10). The production in 2009-10 is 28.38 lac M.T and in 2010-11 20.33 lac M.T (up to dec.’10). Average stripping ratio is 1:4. Total reserve of limestone was 160 lac M.T. The over burden formation was limestone and white clay. The local geology and period, series, formation of lignite and hard rocks at Panandhro lignite mines are comprised.

Based on lignite field characteristics the entire field is divided into four mineable blocks. Overburden, inner burden and lignite is being removed by different mining methods like manual mining, mechanized mining and mining by bucket wheel excavators. The lignite bearing area of Panandhro lignite field has an extent of 8.3 sq km. Because of hard rock as intermediate strata in lignite the rock slope problem take place in mines. The slope stability of hard rocks and lignite is much important issue. A series of landslides occurred as a result of exceptionally heavy rains. These slides caused some loss of life and a significant amount of property damage. Consequently, an extensive review was carried out on the stability of soil and rock slopes in the Territory. A rock slide on a road is caused by the undercutting of sheet joints in a limestone slope. In hard rocks such as limestone, failure can occur very suddenly if the factor of safety of the slope is close to 1.

Figure 1.1: Slope failure at Panandhro Lignite Project
1.4 Panandhro Lignite Project GMDC is running its prestigious project in Lakhpat Taluka in Kutch district. GEB has also established a220 MW thermal power station at Panandhro utilising lignite as fuel. Out of 200 million tonnes reserves of lignite in Kutch, 100 million tonnes are located at Panandhro. The mining activity was started here in 1974 and substantially expansion was under in 1988 till 2012. Now Panandhro Lignite Project is in its last phase.

The lignite is not supplied to thermal power plant but also to 2000 industrial units all over the state. Panandhro lignite field presents a valley base due to surrounding semicircular hill ranges composed of trap. Undulatory terrains with deep ravines at places have been observed within the basin. The maximum and minimum surface reduced levels observed in the basin are 30 meters and 45 meters respectively. A prominent land mark which can be seen even from far off distances is Babia Hill rising to a height of about 90 m. above mean sea level and it is situated on north and north western extremity of the field. The ground level rises in the limestone country with a marked relief and colour. Babia hill itself is composed of different types of limestone. The rest of the area presents variegated colourful topography exhibited by shales, clays and altered traps. The two major rivers viz. Korawadi and Kali pass through the study area, the former from east to west direction and the later from south to north direction. Korawadi River joins Kali river in the western side of the field and finally they drain into sea near Lakhpat. These rivers are torrential in nature and act mostly as flood channels. They remain mostly dry throughout the year excepting during heavy monsoon rains (which is very rare) in their respective catchment areas. There are several small and big creeks which join the two rivers and they also remain dry for the most part of a year. Panandhro dam is in existence on Kaliriver near Panandhro village, a distance of about 10 km and used for drinking and agricultural purposes. The dam also remains dry on many occasions. Godhatad dam on Korawadi River near Dharesi village is mainly used to store the water for drinking and agricultural purposes and to have the desired effect of a check dam.

Origin of lignite: The organic original of coal from peat is undoubted fact. It now seems certain that constant features of the history of the earth, since the origin of the vegetable matter, has been formation of the peat, that is accumulation of plant parts in various stages of degradation by biological as well as chemical processes. Some periods of earth history have been widespread formation of peat at very long period. Subsequent changes have converted it into various type of coal. Alteration of original plant to peat and then into coal is a complex process. There are, of course, biological changes in plant material during peat formation process. These changes may be rather pronounced if the material is unable to decay and the plant material is deposited or the plant parts are decay resistant, biological decomposition can be affected a little. After the peat has been covered with sand clay and other sediments, the coalification changes that occur are not affected by biological processes. The first stage of conversion to peat from coal is simply by compaction due to gravity. This stage is sometimes called as digenesis. Subsequent conversions by which material is converted to coal to various ranks are called metamorphosis. In the course of alteration in the typical lignite, the original peat losses water, which forma about 80-90% of raw peat. Lignite is very reactive organic substance. It combines so readily actively with oxygen in such a way that its storage requires more precaution from spontaneous combustion. The reaction of lignite too many kinds of organic and inorganic solvent is greater than that of bituminous coal. It contains sufficient quantities of carbohydrate derivatives. In Panandhro, lignite deposit occurs at shallow depths without hard strata as roof and Inter burden it is being extracted by opencast methods from 1974. Based on lignite field characteristics the entire field is divided into four mineable blocks. Overburden, inter burden and lignite is being removed by different mining methods like manual mining, mechanised mining and mining by bucket wheel excavators. The lignite bearing area of Panandhro lignite field has an extent of 8.3 sq km. The formations belonging to upper cretaceous period to lower Oligocene period are exposed in an around the field. Lignite in this field appears to be made of several seams separated by bands or intercalations. As many as ten seams have been reported in several bore holes drilled at Panandhro lignite field. The thicknesses of these individual seams vary from a minimum of 0.10 m. to 10.50 m.

Chapter- 2 Literature Review
2.1 Open pit slope- introduction: In open pit mining, mineral deposits are mined from the open surface and downwards. Consequently pit slopes are formed as the ore is being extracted. The pit slope must be inclined so that to avoid slope failures. This angle is controlled by geo mechanical conditions as specific mine and represent an upper bound to the overall slope angle. The actual slope angle depends on (i) Presence of haulage roads, ramps and transportation required for the blasted ore to the pit. (ii) Possible blast damage (iii) Ore grades (iv) Economic constraints Inter-ramp stability can be controlled by structure, by rock mass strength, or both. Inter-ramp heights and rock strengths are the factors controlling failure through the rock mass. Overall stability is usually controlled by rock mass strength and structure, especially in large open pits. Overall stability is always checked for the possibility of deep-seated failure through the rock mass and the method of slices is usually the tool of choice.

2.2 Slope stability: Slope stability is the greatest problem faced by open pit mines. The slope stability in opencast mines is a basic requirement for safety and economic mining operations. Along with other factors, the presence of ground water on the face of mining slopes can often create serious stability problems. This is caused by an increase in fissure water pressure which reduces the effective shear strength and increases unit weight and as a consequence, shears strength. The scale of slope stability is divided into two parts: 1. Gross stability problem: It refers to the large volume of material which comes down the slopes due to large rotational type of shear failure and it involves deeply weathered rock and soil. 2. Local stability problem: This problem refers to much smaller volume of material and these types of materials affect one or two benches at a time due to shear plate jointing, slope erosion due to surface drainage.
Factors affecting slope stability
Slope failures of different types are affected by following factors-
2.2.1 Slope geometry: The basic slope design parameters are height, overall slope angle and area of failure surface. With increase in height the slope stability decreases. The overall slope angle increases the possible extent of development of the any failure of the rear to the crest increases and it should be considered so that ground deformation at the mine can be avoided.

Figure 2.1: Bench, Ramp, Overall angle and their respective angles.

2.2.2 Geological structure:
The main geological structures which affect the slope stability of open pit mines are:
1. Amount and direction of dip.
2. Intra formational shear zones.
3. Joints and discontinuities. (a) Reduce shear strength (b) Range permeability (c) Act as sub surface drain and plain of failures.
4. Faults (a) Weathering and alteration along the faults. (b) Act as ground water conduits. (c) Provides a probable plane of failure

2.2.3 Lithology: The rock material forming the pit slopes determines the rock mass strength modified with discontinuities, faulting, folding, old workings and weathering. Low rock mass strength is characterized by circular, ravelling and rock fall instability like the formation of slopes in massive stability.

2.2.4 Ground water:
It causes the following:
(a) Alters cohesion and frictional parameters.
(b) Reduce the frictional effective stress. Ground water causes increased up thrust and driving water forces and has adverse effect on the stability of slopes. Physical and chemical effect of pure water pressure in joints filling material can thus alter the cohesion and friction of the discontinuity surface. Physical effects of providing uplift on the joint surface reduce the frictional resistances. This will reduce the shearing resistance along the potential failure plane by reducing the effective normal stress acting on it. Physical and chemical effect of the water pressure in the pores of the rock cause a decrease in the compressive strength particularly where confining stress has been reduced. In Panandhro lignite mines, the lignite seams and the sand aquifer are separated by an intervening shale clay formation except in the central part, where they are in direct contact. When the mining operation started in 1978, this sand aquifer was observed to exert upward artesian pressure. With continuous withdrawal of groundwater first by GMDC and subsequently by Gujarat Electric Board (GEB), the water level has gone down. So groundwater conditions in the area are strongly influenced by hydrogeology as well as human interventions. As the area does not receive any significant rainfall, and also the area contains formations which are not good aquifers, occurrences of groundwater is rare. However, it is reported that mild artesian condition exist in the area due to aquifer sand occurring between lignite beds and basement (Trap).Nevertheless, the quantity and quality of artesian water appear to be very poor. Both the surface and subsurface water availability in the area is very poor.

2.2.5 Mining methods and Equipments:
Generally there are four methods of advance in opencast mining:
(1) Strike-cut advancing down the dip.
(2) Strike cut- advancing up the dip
(3) Dip cut- along the strike
(4) Open pit working The use of dip cuts with advance on the strike reduces the length and time that a face is exposed during excavation. Dip cuts with advance oblique to strike may often used to reduce the strata dip into the excavation. Dip cut generally offer the most stable method of working but suffer from restricted production potential. Open pit method are used in steeply dipping seams, due to increased slope height are more prone to large slob/buckling modes of failure.

2.2.6. Dynamic forces: Due to effect of blasting and vibration, shear stresses are momentarily increased and as result dynamic acceleration of material and thus increases the stability problem in the slope face. It causes the ground motion and fracturing of rocks. Blasting is a primary factor governing the maximum achievable bench face angles. The effects of careless or poorly designed blasting can be very significant for bench stability, as noted by Sage (1976) and Bauer and Calder (1971). Besides blast damage and back break which both reduce the bench face angle, vibrations from blasting could potentially cause failure of the rock mass. For small scale slopes, various types of smooth blasting have been proposed to reduce these effects and the experiences are quite good (e.g. Hoek and Bray, 1981). For large scale slopes, however, blasting becomes less of problem since back break and blast damage of benches have negligible effects on the stable overall slope angle. Furthermore, the high frequency of the blast acceleration waves prohibit them from displacing large rock masses uniformly, as pointed out by Bauer and Calder (1971). Blasting-induced failures are thus a marginal problem for large scale slopes. Seismic events, i.e., low frequency vibrations, could be more dangerous for large scale slopes and several seismic-induced failures of natural slopes have been observed in mountainous areas.

2.2.7 Cohesion: It is the characteristic property of a rock or soil that measures how well it resists being deformed or broken by forces such as gravity. In soils/rocks true cohesion is caused by electrostatic forces in stiff over consolidated clays, cementing by Fe2O3, CaCO3, NaCl, etc and root cohesion. However the apparent cohesion is caused by negative capillary pressure and pore pressure response during undrained loading. Slopes having rocks/soils with less cohesion tend to be less stable. The factors that strengthen cohesive force are as follows: 1. Friction 2. Stickiness of particles can hold the soil grains together. However, being too wet or too dry can reduce cohesive strength. 3. Cementation of grains by calcite or silica deposition can solidify earth materials into strong rocks. 4. Man-made reinforcements can prevent some movement of material.

The factors that weaken cohesive strength are as follows: 1. High water content can weaken cohesion because abundant water both lubricates (overcoming friction) and adds weight to a mass. 2. Alternating expansion by wetting and contraction by drying of water reduces strength of cohesion, just like alternating expansion by freezing and contraction by thawing. This repeated expansion is perpendicular to the surface and contraction vertically by gravity overcomes cohesion resulting with the rock and sediment moving slowly downhill. 3. Undercutting in slopes 4. Vibrations from earthquakes, sonic booms, blasting that create vibrations which overcome cohesion and cause mass movement.

2.2.8 Angle of internal friction: Angle of internal friction is the angle (φ ), measured between the normal force (N) and resultant force (R), that is attained when failure just occurs in response to a shearing stress (S). Its tangent (S/N) is the coefficient of sliding friction. It is a measure of the ability of a unit of rock or soil to withstand a shear stress. This is affected by particle roundness and particle size. Lower roundness or larger median particle size results in larger friction angle. It is also affected by quartz content. The sands with less quartz contained greater amounts of potassium-feldspar, plagioclase, calcite, and/or dolomite and these minerals generally have higher sliding frictional resistance compared to that of quartz.

2.3 Types of failure
2.3.1 Planar failure: Simple plane failure is the easiest form of rock slope failure to analyze. It occurs when a discontinuity striking approximately parallel to the slope face and dipping at a lower angle intersects the slope face, enabling the material above the discontinuity to slide. Variations on this simple failure mode can occur when the sliding plane is a combination of joint sets which form a straight path. This means that the solution is never anything more than the analysis of equilibrium of a single block resting on a plane and acted upon by a number of external forces (water pressure, earth quake, etc.) deterministic and probabilistic solution in which parameters are considered as being precisely known may be readily obtained by hand calculation if effect of moment is neglected.

Figure 2.2: Plane failure (after Coates, 1977; Call and Savely, 1990).

For a plane failure analysis, the geometry of the slope is very critically studied. In this connection two cases must be considered:- 1. A slope having tension crack in the upper face. 2. A slope with tension crack in the slope face.

General condition for planar failure: a) The plane on which sliding occurs must strike parallel or nearly parallel to the slope. b) The sliding plane must daylight in the slope face, which means that the dip of the plane must be less than the dip of the slope face, that is, ψp < ψf . c) The dip of the sliding plane must be greater than the angle of friction of this plane, that is, ψp > φ. d) The upper end of the sliding surface either intersects the upper slope, or terminates in a tension crack. e) Release surfaces that provide negligible resistance to sliding must be present in the rock mass to define the lateral boundaries of the slide. Alternatively, failure can occur on a sliding plane passing through the convex “nose” of a slope. In the absence of actual ground water pressure measurements within a slope, the current state of knowledge in rock engineering does not permit a precise definition of the ground water flow patterns in a rock mass. Consequently, slope design should assess the sensitivity of the factor of safety to a range of realistic ground water pressures, and particularly the effects of transient pressures due to rapid recharge. The following are four possible ground water conditions that may occur in rock slopes, and the equations that can be used to calculate the water forces U and V. In these examples, the pressure distributions in the tension crack and along the sliding plane are idealized and judgment is required to determine the most suitable condition for any particular slope. a) Ground water level is above the base of tension crack so water pressures act both in the tension crack and on the sliding plane. If the water discharges to the atmosphere where the sliding place daylights on the slope face, then it is assumed that the pressure decreases linearly from the base of the tension crack to zero at the face. b) Water pressure may develop in the tension crack only, in conditions for example, where a heavy rainstorm after a long dry spell results in surface water flowing directly into the crack. If the remainder of the rock mass is relatively impermeable, or the sliding surface contains a low permeability clay filling, then the uplift force U could also be zero or nearly zero. c) Ground water discharge at the face may be blocked by freezing. Where the frost penetrates only a few meters behind the face, water pressures can build up in the slope and the uplift pressure U can exceed. d) Ground water level in the slope is below the base of the tension crack so water pressure acts only on the sliding plane.

2.3.2 Wedge failure:
The three dimensional wedge failures occur when two discontinuities intersects in such a way that the wedge of material, formed above the discontinuities, can slide out in a direction parallel to the line of intersection of the two discontinuities. It is particularly common in the individual bench scale but can also provide the failure mechanism for a large slope where structures are very continuous and extensive. Figure 2.3: Wedge failure (after Hoek and Bray, 1981) When two discontinuities strike obliquely across the slope face and their line of intersection ‘daylights’ in the slope, the wedge of the rock resting over these discontinuities will slide down along the line of intersection provided the inclination of these line is significantly greater than the angle of friction and the shearing component of the plane of the discontinuities is less than - 15 - the total downward force. The total downward force is the downward component of the weight of the wedge and the external forces (surcharges) acting over the wedge.
Based on this geometry, the general conditions for wedge failure are as follows: a) Two planes will always intersect in a line. b) The plunge of the line of intersection must be flatter than the dip of the face, and steeper than the average friction angle of the two slide planes. c) The line of intersection must dip in a direction out of the face for sliding to be feasible.

2.3.3 Circular failure: The pioneering work, in the beginning of the century, in Sweden confirmed that the surface of the failure in spoil dumps or soil slopes resembles the shape of a circular arc. This failure can occurs in soil slopes, the circular method occurs when the joint sets are not very well defined. When the material of the spoil dump slopes are weak such as soil, heavily jointed or broken rock mass, the failure is defined by a single discontinuity surface but will tend to follow a circular path.

The conditions under which circular failure occurs are follows: 1. When the individual particles of soil or rock mass, comprising the slopes are small as compared to the slope. 2. When the particles are not locked as a result of their shape and tend to behave as soil. Figure 2.4: Three-dimensional failure geometry of a rotational shear failure (after Hoek and Bray, 1981).

2.3.3.1 Types of circular failure Circular failure is classified in three types depending on the area that is affected by the failure surface. They are:- 1. Slope failure: In this type of failure, the arc of the rupture surface meets the slope above the toe of the slope. This happens when the slope angle is very high and the soil close to the toe posses the high strength. 2. Toe failure: In this type of failure, the arc of the rupture surface meets the slope at the toe. 3. Base failure: In this type of failure, the arc of the failure passes below the toe and in to base of the slope. This happens when the slope angle is low and the soil below the base is softer and more plastic than the soil above the base. The actual shape of the “circular” slide surface is influenced by the geological conditions in the slope. For example, in a homogenous weak or weathered rock mass, or a rock fill, the failure is likely to form as a shallow, large radius surface extending from a tension crack close behind the crest to the toe of the slope. This contrasts with failures in high cohesion, low friction materials such as clays where the surface may be deeper with a smaller radius that may exit beyond the toe of the slope.

2.3.4 Two Block Failure: Two block failures are much less common mode of rock slope failure than single block failures such as the planes and the 3D wedge and, consequently, are only briefly considered here. Several methods of solution exist and each may be appropriate at some level of investigation.

2.3.5 Toppling Failure: Toppling or overturning has been recognized by several investigators as being a mechanism of rock slope failure and has been postulated as the cause of several failures ranging from small to large ones.

Figure 2.5: Toppling failure It occurs in slopes having near vertical joint sets very often the stability depends on the stability of one or two key blocks. Once they are disturbed the system may collapse or this failure has been postulated as the cause of several failures ranging from small to large size. This type of failure involves rotation of blocks of rocks about some fixed base. This type of failure generally occurred when the hill slopes are very steep.

2.3.5.1 Types of toppling failure

Goodman and Bray (1976) have described a number of different types of toppling failures that may be encountered in the field, and each is discussed briefly on the following pages. The importance of distinguishing between types of toppling is that there are two distinct methods of stability analysis for toppling failures as described in the following pages—block and flexural toppling— and it is necessary to use the appropriate analysis in design.

Block toppling

Block toppling occurs when, in strong rock, individual columns are formed by a set of discontinuities dipping steeply into the face, and a second set of widely spaced orthogonal joints defines the column height. The short columns forming the toe of the slope are pushed forward by the loads from the longer overturning columns behind, and this sliding of the toe allows further toppling to develop higher up the slope. The base of the failure generally consists of a stepped surface rising from one cross joint to the next. Typical geological conditions in which this type of failure may occur are bedded sandstone and columnar basalt in which orthogonal jointing is well developed.

Flexural toppling

Continuous columns of rock, separated by well developed, steeply dipping discontinuities, breaking in flexure as they bend forward. Typical geological conditions in which this type of failure may occur are thinly bedded shale and slate in which orthogonal jointing is not well developed. Generally, the basal plane of a flexural topple is not as well defined as a block topple.

Block-flexure toppling Block-flexure toppling is characterized by pseudo-continuous flexure along long columns that are divided by numerous cross joints. Instead of the flexural failure of continuous columns resulting in flexural toppling, toppling of columns in this case results from accumulated displacements on the cross-joints. Because of the large number of small movements in this type of topple, there are fewer tension cracks than in flexural toppling, and fewer edge-to-face contacts and voids than in block toppling.

2.4 Factors to be considered in the assessment of stability
2.4.1 Ground water investigation Before any further examination of an existing slope, or the ground on which a slope is to be built, essential borehole information must be obtained. This information will give details of the strata, moisture content and the standing water level and shear planes. Piezometer tubes are installed into the ground to measure changes in water level over a period of time. Ground investigations also include: * In-situ and laboratory tests. * Aerial photographs. * Study of geological maps and memoirs to indicate probable soil conditions. * Visiting and observing the slope.

2.4.2 Most critical failure surface: In homogeneous soils relatively unaffected by faults or bedding, deep seated shear failure surfaces tend to form in a circular, rotational manner. The aim is to find the most critical surface using "trial circles". The method is as follows: 1. A series of slip circles of different radii is to be considered but with same centre of rotation. Factor of Safety (FOS) for each of these circles is plotted against radius, and the minimum FOS is found. 2. This should be repeated for several circles, each investigated from an array of centres. The simplest way to do this is to form a rectangular grid from the centres. Each centre will have a minimum FOS and the overall lowest FOS from all the centre shows that FOS for the whole slope. This assumes that enough circles, with a large spread of radii, and a large grid of centres have been investigated.
An overall failure surface is found.

2.4.3 Tension cracks: A tension crack at the head of a slide suggests strongly that instability is imminent. Tension cracks are sometimes used in slope stability calculations, and sometimes they are considered to be full of water. Tension cracks are not usually important in stability analysis, but can become so in some special cases. Therefore assume that the cracks don't occur, but take account of them in analyzing a slope which has already cracked.

2.4.4 Submerged slopes: When an external water load is applied to a slope, the pressure it exerts tends to have a stabilizing effect on the slope. The vertical and horizontal forces due to the water must be taken into account in analysis of the slope. Thus, allowing for the external water forces by using submerged densities in the slope, and by ignoring water externally.

2.4.5 Factor of safety: The FOS is chosen as a ratio of the available shear strength to that required to keep the slope stable.

Factor of safety | Details of slope | < 1.0 | Unsafe | 1.0-1.25 | Questionable safety | 1.26-1.4 | Satisfactory for routine cuts and fills,Questionable for dams, or wherefailure would be catastrophic | >1.4 | Satisfactory for dams |
Table 2.1 Guidelines for equilibrium of a slope

For highly unlikely loading conditions, factors of safety can be as low as 1.2-1.25, even for dams e.g. situations based on seismic effects, or where there is rapid drawdown of the water level in a reservoir.

2.4.6 Progressive Failure: This is the term describing the condition when different parts of a failure surface reach failure at different times. This often occurs if a potential failure surface passes through a foundation material which is fissured or has joints or pre-existing failure surfaces. Where these fissures occur there will be large strain values, so the peak shear strength is reached before other places.

2.4.7 Pre-Existing Failure Surfaces: If the foundation on which a slope sits contains pre-existing failure surfaces, there is a large possibility that progressive failure will take place if another failure surface were to cut through them. The way to deal with this situation is to assume that sufficient movement has previously taken place for the ultimate state to develop in the soil and then using the ultimate state parameters. If failure has not taken place, then a decision has to be made on which parameters to be used.

2.5 Methods of analysis
2.5.1 Wedge failure analysis The 3D nature of the wedge failure analysis complicates the analysis. The different methods of analysis are given as follows:

2.5.1.1 Spherical projection solution using factor of safety

The 3D wedge problem can be very easily analysed using spherical projection techniques. When the shear strength of the shear surface is entirely frictional and there is no external force, the problem becomes dimensionless and can be analyzed very simply by the means of a stereo net analysis alone. The introduction of water pressure or the external forces requires the use of side calculations to determine the orientation of the resultant forces acting on the wedge. Use of spherical projection rapidly establishes a zone of orientations for the resultant force for which the wedge will remain stable. The orientation of the line of intersection of the wedge is defined by the intersection of the great circles which defines the joints. To determine the factor of safety against sliding, the great circle containing both the resultant force acting on the wedge and the resultant shear force is drawn. The intersection of this great circle with and through both the normal and both the reactions on the shear planes define the position of the resultant of these normal and reactions. The factor of safety can be defined as the ratio of the resultant shear force acting along the line of intersection of the wedge to the resultant shear strength available to resist sliding in the same direction.

2.5.1.2 Chart solution Hoek and Bray (1980) produced a series of charts which can be used to rapidly access the stability of rock wedges for which there is known cohesion or external forces. Under these condition and for a given friction angle, the factor of safety is a function only of the dip and direction of the shear plane. These charts are convenient to use for use simple wedge problem but suffer from the disadvantage that it does not give the feel of the problem.

2.5.1.3 Spherical Projections Solutions using Probabilistic Approach Monte Carlo analysis of the wedge failure gives, with a specified confidence level, the uncertainty in the orientations of the shear planes. When the orientations of the shear planes are known then the spherical projection technique can be used to find out the orientation of the failure plane.

2.5.2 Circular Failure Analysis The stability of the slopes of finite extent like that in the case of circular is analyzed by the method of dividing the whole suspected failure area in to slices and further analyzing the sequence of events that may follow thereafter. There are several methods of slices in their new advancement together with friction circle method and tailors stability number method.

2.5.2.1 Method of Slices This method was advanced by the Swedish geotechnical commission and developed by W.Fellienius (1936). By dividing the mass above an assumed rupture surface of failure in to vertical slices and assuming that the forces on the opposite sides of each slice are equal and opposite, a statistically determinate problem is obtained and semi graphical method have been devised by which the stability of the mass may be analyzed for any given circle. The main objection of this method is that the most dangerous of infinite number of circles are to be found out for which graphical method is to be used for a number of time.

2.5.2.2 Modified Method of Slices When there are several dangerous circles to be analyzed usual procedure by the slice method is quite tedious. N.C. Coutrney of U.S.A. has developed simple graphical solutions by which the forces that are inherent in the method of slices such as the forces acting on the vertical sides of the slices.

2.5.2.3 Simplified Method of Slices This method takes in to account the forces acting on the vertical sides of the slices in the development of an equation for determining the factor of safety. However, the simplified equation proposed by Bishop (1955) does not contain the forces acting on the vertical sides and there by simplifies the computation.

2.5.2.4 Friction Circle Method It is a very convenient method which takes in to account the total forces acting on the whole mass lying above the assumed circular surface of failure. This method eliminates the indeterminate forces that are inherent in the method of slices such as acting on the vertical sides of the slices.

2.5.2.5 Taylor’s Stability Number Taylor (1937) made a mathematical trial method using the friction circle method. Charts as formulated by Taylor give the relationship between stability number and the slope angle for various angle of friction. This method is applicable to homogeneous simple slopes without seepage.

2.5.3 Two Block Failure Analysis
2.5.3.1 Stereographic Solution A stereographic analysis is convenient way of determining whether or not a two block configuration will stable (Goodman, 1975 and Kuykendall and Goodman, 1976). Any form of shear strength envelope can be accounted for by use of the secant angle of friction.

2.5.4 Toppling Failure Analysis Base friction models can be useful insight in to the mechanism of failure. They can also be used to provide a quantitative assessment of the effect of possible slope stabilization procedure such as reducing the slope angle or installing horizontal reinforcements. The difference conditions are taken in to account to ascertain sliding and toppling of block in inclined plane.

2.5.5 Other Methods of Analysis
2.5.5.1 Limit Equilibrium Method In limit equilibrium method of analysis, static force is applied to analyze the stability of the rock mass or soil above the failure surface. If failure has already occurred, the geometry of the failure surface can be determined and the analysis of the failure can be done and is known as back analysis. If it is a design situation, however the failure surface is potential rather than actual, many potential surface may have to be analyzed to find the critical geometry before an acceptable slope geometry can be accounted for. In the case of plane failure, 3D wedge failure, circular failure, the material above the failure surface will be on the point of slipping when the disturbing forces due to gravity are just counterbalanced by the forces tending to restore equilibrium. The ratio of the two forces defines the factor of safety of the failure surface.

2.5.5.2 Stress Analysis Method

Failure does not necessarily occur along a well defined failure surfaces. The situation where the structural condition does not permit sliding along the discontinuity surface, crushing of the rock occurs at the points of the highest stress. Progressive failure of the rock mass can subsequently deform the slope and may cause the catastrophic failure. The objectives of the stress analysis method are to represent the rock mass by a series of structural elements (finite element method) or cells of constraint of materials (one finite different method) and perform an analysis to determine to stresses at points within the slope. The stress distribution can be examined to determine where rock failure is likely to occur, rock failure occurs when the stresses to which the rock is subjected more than its strength.

Conclusion Opencast mining is a very cost-effective mining method allowing a high grade of mechanization and large production volumes. Mining depths in open pits have increased steadily during the last decade which has the increased risk of large scale stability problems. It is necessary to assess the different types of slope failure and take cost effective suitable measures to prevent, eliminate and minimize risk. The parametric study which was carried by varying the cohesion, angle of internal friction and ultimate slope angle showed that with increase in ultimate slope angle, the factor of safety decreases. Moreover cohesion and angle of internal friction are quite important factors affecting slope stability. With increase in both the parameters the stability increases. Conduct of slope stability assessment in Indian mines is mostly based on empirical and observational approach; hence effort is made by statutory bodies to have more application of analytical numerical modelling in this field to make slope assessment and design scientific. This will ensure that suitable corrective actions can be taken in a timely manner to minimize the slope failures and the associated risks.

References 1. Hoek, E. & Bray, J.W. (1980), “Rock Slope Engineering”, Institute of Mining & Metallurgy, London, pp.45-67.